As Barack Obama’s second term begins with a private swearing-in ceremony today, we know one thing: If he leaves office as a well-regarded two-termer in 2017, he will have become the most important president since Ronald Reagan and the most important liberal president since Franklin D. Roosevelt.

If the American electorate deems his eight years a success, the Obama presidency will have altered the ideological trajectory of the United States.

For decades now, since the Reagan era, the United States has been most accurately described as a center-right nation. That will not be the case if Obama does well over the next four years. By 2017, it will have moved several clicks port — for a generation at least. Due to Obama and his policies, the United States will have become a center-left nation.

If, however, Obama crashes and burns during his second term — either because the public turns on his policies, as was the case with George W. Bush, or (impossible to believe) because of personal misconduct, as was the case with Bill Clinton — Obama will leave liberalism in the same state of crisis in which conservatism now finds itself.

Rather than enshrine it as the dominating force in American politics, he will have discredited left-liberalism.

So the stakes are pretty high. The reward for success will be vast. The cost of failure will be severe. And Obama has made sure we will be able to tell which is which. There will be no doubt.

He’s not Clinton, who saved his presidency by tacking to the right after losing in 1994. He’s not George W. Bush, who passed an education bill with Teddy Kennedy’s help and created a new entitlement program in 2003 (the Medicare prescription-drug benefit). Both presidents were loathed unreservedly by their ideological opposites, and yet both frequently tacked to the center on domestic politics.

By contrast, Obama was the first successful candidate for the presidency to call himself a “liberal” with no discomfort since Lyndon Johnson in 1964. And he meant it. His significant legislative accomplishments, all of which have tacked to the left, would have seemed like science-fictional dreams in 2005, when it appeared liberalism had been routed and conservatism had embedded itself permanently throughout the government.

Obama’s first term began with the passage of the largest domestic spending bill in US history, the trillion-dollar stimulus.

He moved on post-haste to the partial nationalization of two auto manufacturers, an unprecedented reach into the private marketplace made all the more audacious for its naked giveaways to the same auto workers union that played such a role in leading the companies to ruin.

Then came his most important achievement, the passage of ObamaCare after a year’s debate — a national health plan, the great liberal desiderata for decades and decades.

Obama paid for it, losing 63 seats in the House in the 2010 elections. But it turned out there was political genius in the design of ObamaCare — it is so complex, involving so many moving parts, that the implementation of its most intrusive and problematic aspects had to be delayed until 2014.

So it caused no short-term pain, and did not turn anyone new against it who had not been against it in 2010. The most controversial aspect of his presidency played no role in his reelection.

But now it will — as will everything else on which he got a pass from the electorate who brought him back for a second term. And here’s where he may founder.

The national debt stands at $16 trillion, nearly $6 trillion more than in 2008, and it’s growing.

The nation is going to have to digest ObamaCare, and the results may be extraordinarily ugly. Tens of thousands of doctors are going to retire rather than subject themselves to a more suffocating regulatory yoke than the one they already suffer under.

There will be a pitched war between the states and the federal government on the regulations governing the purchase of health insurance. Costs for individuals and their employers will go up; after all, someone has to pay for the 30 million uninsured who will now be covered.

The cure for these woes is, simply, wide-ranging and sustained economic growth at levels far higher than today — which will boost employment, fill government coffers, and offer real optimism about the future that will make the digestion of ObamaCare more palatable.

But Obama’s brand of liberalism, as the past four years have shown, is not conducive to fast growth — GDP growth, job growth, any growth except the growth of government regulation.

Job growth is ongoing, but it’s anemic, and may remain anemic because employers are going to resist loading themselves up with full-time workers with all these newly mandated health-care costs.

And it’s not keeping up with national needs. The same number of people overall were employed in 2012 as in 2008 — meaning that there are many millions of able-bodied workers who no longer have a connection to the workforce at all.

That bodes ill, to put it mildly, as does the rapidity with which the national debt is growing and the utter lack of interest the president expresses in taking actions to slow or halt or reverse it.

This is Obama’s America now. He won it twice; he owns it. If he fixes it, it will be his, effectively, for decades. But if he turns it over to the next guy in roughly the same condition it’s in today, Barack Obama will have proved definitively that liberalism simply doesn’t work.